Electrospray ionization (ESI) mass spectrometry (MS) is now widely used for analyses of biomolecules, pharmaceutical compounds, synthetic polymers and so on (N. Ceck, et al., Mass Spectrom. Rev. 2001, 20, 362–387). Great needs to enhanced sensitivity and better connectivity for capillary high-performance liquid chromatography (HPLC), especially in proteomics and metabolomics, have led to ESI operation in the low flow rate range of less than 1 μl/min. This was achieved using tapered capillary emitters with a very small outlet orifice, usually made of glass and fused silica with or without either metal or conducting-polymer coating, and of stainless steel (D. Gale, et al., Rapid Commun. Mass Spectrom. 1993, 7, 1017–1021; M. Emmett, et al., J. Am. Soc. Mass Spectrom. 1994, 5, 605–613; G. Valaskovic, et al., Anal. Chem. 1995, 67, 3802–3805; M. Wilm, et al., Anal. Chem. 1996, 68, 1–8; E. Maziarz III, et al., J. Am. Soc. Mass Spectrom. 2000, 11, 659–663; and Y. Ishihama, et al., Rapid Commun. Mass Spectrom. 2002, 16, 913–918). Practice with these emitters have, however, revealed problems, such as, easy clogging at their exit orifices, and instability of spraying water-rich solution of a high surface tension at an early stage of gradient elution on reverse-phase HPLC being frequently used for proteomics and metabolomics.
These problems are mutually related and arise primarily from the high surface energy (γL and γS, respectively) of both aqueous solutions and materials the emitters are made of. Electrospray initiates from expansion of liquid into a dynamic cone with the charged surface (Taylor cone) at the capillary exit by the applied high electric field (for review ref. 1 and references cited therein). When the force of the applied field on and Coulombic repulsion of the surface charges surpass the surface tension of solution, droplets with excess charge of a given sign detach from the cone. Higher surface energy of the tip material (γS=about 100 mN/m for glass) produces a cone supported by the base of a larger diameter on the outer wall of the tip (FIG. 1A), leading to a lower charge density on the cone surface, because the rate of excess charge production at the electric contact site of the emitter is rather constant. This causes difficulty in spraying aqueous solutions of the high surface tension (γL=73 mN/m), and eventually a need to extremely sharp emitter tips.